1. Field of the Invention
An aspect of the present invention relates in general to an optical recording/reproducing apparatus, and more specifically to an optical pickup actuator performing tilting control of a lens holder on which a plurality of object lenses are mounted, and an optical recording/reproducing apparatus having the same.
2. Description of the Related Art
Generally, an optical pickup for recording and/or reproducing information onto and/or from an optical medium such as a CD (Compact Disk) or a DVD (Digital Versatile Disk) is provided with an optical system including an object lens for focusing light from a light source onto the optical medium to record/reproduce information onto/from the optical medium, and an optical pickup actuator for adjusting a distance between the object lens and the optical medium and a position and a tilt of the object lens relative to the optical medium.
The optical pickup actuator performs focusing control, tracking control, and tilting control of the object lens. The focusing control is performed to maintain a fixed distance between the optical medium and the object lens. The tracking control is performed to control a position of the object lens in a radial direction of the object medium so that the object lens follows a desired track on the optical medium. Lastly, the tilting control is performed to tilt the object lens so that light passing through the object lens is perpendicular to a surface of the optical medium. Particularly, the tilting control is required to reduce errors in information to be recorded/reproduced due to aberrations caused by a tilt in the object lens relative to the optical medium.
In tilting control, when one object lens 10 is used in an optical pickup as shown in FIGS. 1A and 1B, the object lens 10 is arranged at a central axis CL of a lens holder 12, and the lens holder 12 is tilted about the central axis CL. Therefore, although the lens holder 12 may be tilted, an offset of the object lens is not generated in the focusing direction FD and the tracking direction TRD.
FIGS. 2A and 2B show a newer type of optical pickup actuator in which a plurality of object lenses are mounted on a lens holder for use with optical media D having different recording densities.
In FIGS. 2A and 2B, two object lenses 20, 21 are mounted in parallel on one lens holder 22 in the tracking direction TRD. When the lens holder 22 is tilted about its central axis CL as shown in FIG. 2B, an offset is generated in the focusing direction FD and the tracking direction TRD with respect to both of the object lenses 20, 21. This offset is computed as follows.OLF=d sin θOLT=d(1−cos θ)  [Equation 1]
Here, OLF indicates an offset amount of the object lenses 20, 21 in the focusing direction FD, and OLT indicates an offset amount of the object lenses 20, 21 in the tracking direction TRD. In addition, d indicates a distance from the central axis CL of the lens holder 22 to optical axes of the object lenses 20, 21, and θ indicates a tilting angle of the lens holder 22. Here, it is assumed that the distance d from the central axis CL of the lens holder 22 to the optical axes of the object lenses 20, 21 is fixed. In the majority of cases, however, the distance between the optical axis of the object lens 20 and the central axis CL of the lens holder 22 is different from the distance between the optical axis of the object lens 21 and the central axis CL of the lens holder 22.
When the object lens offset is generated, it is necessary to shift the object lenses 20, 21 by the offset amount OLF in the focusing direction FD, and by the offset amount OLT in the tracking direction TRD.
In other words, tilting control is always accompanied by focusing control and tracking control. Unfortunately, however, this mechanism only increases a processing load on a controller (not shown) which is in charge of all the controls.
Moreover, as a result of the shifting of the object lenses in the focusing direction FD by the offset amount OLF and in the tracking direction TRD by the offset amount OLT necessitated by the tilting control, a focusing margin (a space in which the lens holder 22 can move in the focusing direction FD for focusing control of the object lenses 20, 21) and a tracking margin (a space in which the lens holder 22 can move in the tracking direction TRD for tracking control of object lenses 20, 21) are reduced. As expressed in Equation 1, the offset amount OLF and the offset amount OLT increase in proportion to an increase in the tilting angle (θ) of the lens holder 22, so that at an extreme tilting angle, it may not be possible to perform focusing control and tracking control. In addition, if the optical medium D is severely deformed, the lens holder 22 and the optical medium D may collide with each other. Unfortunately, these problems have grown more serious with the development of a high-density (HD) optical medium D because the distance between the object lenses 20, 21 and the HD optical medium D is even shorter than it is for an earlier kind of optical medium D.